1. Field of the Invention
The invention relates to a magnetic resonance (MR) device which is provided with a medical instrument which is to be introduced into an object to be examined, and also with a coil system which is arranged in or on the instrument and includes at least one coil for receiving and/or transmitting an RF signal. The invention also relates to a medical instrument to be introduced into an object to be examined, notably a catheter or an endoscope, provided with a coil system which is arranged in or on the instrument and includes at least one coil for receiving and/or transmitting an RF signal, as well as to a method of determining the position of a medical instrument that can be introduced into an object to be examined.
2. Description of Related Art
Such an MR device, medical instrument and method of determining the position thereof are known from U.S. Pat. No. 5,353, 795. Therein, a small RF coil, being a so-called microcoil, is arranged in a catheter which is introduced into a patient. During operation of the MR device, an RF signal is induced into the microcoil after excitation of the examination zone, which RF signal is applied, via an RF lead, to a receiver device which processes the signal and determines the position of the coil. The position can be superposed on an image, for example an MR image or a computer tomography (CT) image.
It has been found that the RF lead required between the transmitter and/or receiver device, arranged outside the object to be examined, to a microcoil which is preferably attached to the tip of the medical instrument constitutes a drawback. There is a risk of heating of tissue due to resonance (notably X/4 resonance) occurring in the vicinity of the RF lead during the transmission phase (RF excitation) of the MR examination. Furthermore, the RF lead must inherently be formed by very thin wires, because the medical instrument must also be introduced into very thin veins for various applications. Consequently, substantial signal losses may occur during the transmission of the signal received by the microcoil to a receiving device. Moreover, in conjunction with the RF lead the microcoil must form a stable resonant circuit; therefore, the length of the RF lead cannot be changed at random. Moreover, the known device requires a separate receiving channel for the microcoil or (when a single receiving channel is used) a switching device is required for switching over between the receiving coil system and the microcoil.